We propose to study the effects of a tumor burden on the natural and acquired resistance of mice to selected experimentally-induced infections. The hosts chosen for study are the immunodeficient athymic nude (nu/nu) mouse and its immunocompetent heterozygote (nu/+) littermate. We believe that these experimental animal models may apply to the human cancer problem. The tumors to be employed are the Methylcolanthene induced (Meth A/B) fibrosarcoma and mouse mammary carcinoma D7041. The infectious agents to be used are: Streptococcus pneumoniae, Salmonella typhimurium, Listeria monocytogenes and ectromelia virus. Specifically we propose: 1) to determine quantitatively alterations in the innate resistance of tumor-bearing mice to the above infectious agents, 2) to determine quantitatively, the capacity of tumor-bearing animals to be specifically or non-specifically immunized against the above infectious agents, and 3) to examine selected immunological parameters (cellular and humoral) which might reflect an altered host resistance conditioned by the neoplasm.